1. Field of the Invention
The present invention relates generally to a small twin spool gas turbine engine, and more specifically to a twin spool rotor shaft assembly for a small twin spool gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine is a very efficient power plant and is used to power aircraft such as a commercial or military aircraft or an unmanned aero vehicle (UAV). The PW4000 series engine made by Pratt & Whitney Aircraft Group is a large commercial turbofan engine with a dual-shaft (twin spool) and high bypass ratio front fan. This engine produces 60,000 pounds of thrust and weighs 9,200 pounds dry. It is a very efficient engine. A twin spool engine has about twice the efficiency of a single spool engine and therefore is used when efficiency is an important factor.
Recent developments in small unmanned aircraft, such as a UAV, have led to the use of small gas turbine engines to power these small aircraft. The more efficient the engine is in the UAV, the longer will be the loiter time. Small single spool gas turbine engines have been around for years to power small radio controlled planes or even a UAV such as a cruise missile or a video surveillance aircraft. With the demand for longer loiter times, a more fuel efficient gas turbine engine is desirable.
It has been common in the art of gas turbine engine design to scale down larger engines to the size needed. A small UAV only requires a few hundred pounds of thrust to power the aircraft. One major problem in the design of small gas turbine engines is scaling the larger engine down to the smaller size. A larger engine can only be scaled down so far before design problems start to surface, such as problems with the critical rotation speed of the shaft becoming lower than the operational speed of the shaft. As the size of the engine decreases, the rotational speed of the rotor shaft must increase in order to retain the high overall efficiency for a gas turbine engine. As the rotor shaft speed increases, rotor dynamic issues can become a major problem. A large gas turbine engine such as the PW4000 series engine cannot be scaled down below a certain size before the rotor dynamics and natural frequency problems start to cause problems. The low speed rotor shaft in the PW4000 series engine can be scaled down only so far before the bending mode (third mode) of the rotor shaft coincides with the natural frequency of the shaft. This is referred to in rotor dynamics as the critical speed. At critical speed, a rotating shaft would break apart from the high bending stresses developed. The bending displacement would theoretically be infinite without adequate damping of the bearings. Engineers attempt to design rotor shaft systems to operate below the critical speed to avoid rotor dynamic caused problems. Thus, the original design for a rotor shaft used in the larger prior art gas turbine engine would not function at the smaller size because the shaft operating speed would be larger than the critical speed and therefore making the smaller scaled down engine inoperable.
In a twin spool gas turbine engine, a low pressure fan or compressor and a low pressure turbine are rotatably attached to the inner or low speed rotor shaft. A high pressure compressor and high pressure turbine are rotatably attached to the outer or high speed rotor shaft. Each shaft is rotatably supported by bearings on the shaft ends. The natural frequency of a rotating shaft is directly proportional to a ratio of the stiffness to the mass of the shaft. Thus, the natural frequency of the rotating shaft can be increased by either increasing the stiffness of the shaft, decreasing the mass of the shaft, or both.
If the shaft is lengthened and thus the distance between the bearings, the natural frequency will be lowered. Thus, to produce a small fuel efficient gas turbine engine useful for a UAV or other small aircraft with a thrust less than 300 pounds, a new design is required for the inner and the outer rotor shafts to make such an engine operational. The inner and outer rotor shafts in a twin spool gas turbine engine requires a new design for each shaft since the prior art twin spool engines cannot be scaled down to this level without rotor dynamics problems occurring that limit the size of the shaft.
U.S. Pat. No. 5,454,222 issued to Dev on Oct. 3, 1995 and entitled SMALL GAS TURBINE ENGINE HAVING ENHANCED FUEL ECONOMY discloses a gas turbine engine having twin spools that has a smaller size and lower weight than existing turbine engines having the same power (at the time of the Dev invention). The core engine has a diameter of about 0.35 meters (about 14 inches) that operates at about 54,000 rpm. As can be seen from the figures in the Dev patent, the inner or low speed rotor shaft is a straight solid shaft while the outer rotor shaft is somewhat curved to conform to the radial compressor and turbine on the shaft. The twin spool engine of the Dev patent (a typical of the prior art twin spool gas turbine engines) cannot be scaled down any further because the rotor shafts would have to operate at higher speeds which would then produce the rotor dynamics problems discussed above.
It is an object of the present invention to provide for a small twin spool gas turbine engine.
Another object of the present invention is to provide for a twin spool rotor shaft assembly in a small gas turbine engine.
Another object of the present invention is to provide for a twin spool rotor shaft assembly for a small gas turbine engine having a hollow inner shaft with a preload applied to the ends of the hollow inner shaft in order to prevent the shafts ends from loosening during engine operation.
Another object of the present invention is to provide for a twin spool rotor shaft assembly for a small gas turbine engine with a cooling air passage from the high pressure outlet to the rim cavity forward of the low pressure turbine in order to limit the injection of hot gas.
Another object of the present invention is to provide for a twin spool rotor shaft assembly for a small gas turbine engine with minimal axial spacing between the low speed rotor shaft bearings in order to maximize the critical speed for the low speed rotor shaft.